Do a lot of flatwork. Michel Robert has published a great set of cards with excercises on them to inspire you. Also keep monitoring your young horse and do not overdo training. Keep a lot of variation in their work (hacking, dressage practice, polework and some jumping and lunging during the week)
Keep practicing alot so they don't forget over a wekk span!
Take it slow even on those days your horse feels like it is doing so well that you can go from poles on the ground to cross rails or from cross rails to a 2' jump. It is muscle and mentality that you are building.
Use guide poles to encourage your horse to jump and not run out